I have run tests with my Duramax and Toyota Corolla with oils that claim to increase mileage. You could never prove it by me. A five to eight PSI change in tire pressure did thought in fact a couple MPG!

I have done 40 plus runs with checking my gas mileage new car old oil.

New oil= 29.4 to 29.8

Old oil= 28.4 to 28.8

And it was remarkably consistent. Every time past 4k miles running Pennzoil Ultra or Castrol gold bottle 5w30. Or around 2700 miles for other group 3 oils. Same road on Rte 5.

Doing something once, twice or three times is not meaningful to me. But having done it 40 plus times... It is and was shockingly consistent fall off. Pretty meaningful to me due to how many times I did it and how consistent it was as well.

Strange but a consistent pattern in both my Ford Fusion and my Nissan Altima 3.5. An interesting note is that Pennzoil Ultra and Castrol gold bottle both held up maximum gas mileage longer than any other oil I ran in either car.

I have done 40 plus runs with checking my gas mileage new car old oil.

New oil= 29.4 to 29.8

Old oil= 28.4 to 28.8

And it was remarkably consistent. Every time past 4k miles running Pennzoil Ultra or Castrol gold bottle 5w30. Or around 2700 miles for other group 3 oils. Same road on Rte 5.

Doing something once, twice or three times is not meaningful to me. But having done it 40 plus times... It is and was shockingly consistent fall off. Pretty meaningful to me due to how many times I did it and how consistent it was as well.

New oil= 29.4 to 29.8 Old oil= 28.4 to 28.8And it was remarkably consistent. Every time past 4k miles

How odd..... I would not have guessed that.4,000 mile oil is usually still in tip-top shape. (ESPECIALLY Pennz U or Castrol GB)

I believe Chris142 has indicated that, in order to pass CA smog, they often recommend an oil change. The new oil is better able to absorb blowby it seems, and thus engine out emissions is improved. If so, then I wonder: perhaps once the oil "loads up" with these lighter compounds, and no longer absorbs it, then any and all blowby is simply rerouted back into the engine. But this blowby now acts as "negative BTU's", or causes ignition timing to be pulled slightly, something, which causes a net loss of power (and thus an increase in fuel consumption). The oil could very well be still very good at what it does primarily (lubricate stuff).